Canon Speedlite 600EX II-RT Manual (en)

: Wireless Multiple-Flash Photography with Flash Ratio
Set the flash exposure 
compensation amount for slave 
unit C.
Set the amount as necessary.
Press function button 3 <F>.
Turn <9>, then select <h>.
Press function button 3 <B>.
Turn <9> to set the flash exposure 
compensation amount, then press 
If you need more flash output or wish to 
perform more sophisticated lighting, you 
can increase the number of slave units. 
Simply set an additional slave unit to the 
firing group (A, B, or C) whose flash 
output you want to increase. You can 
increase the number of slave units up to 
15 units.
For example, if you set a firing group with three slave units to <f>, 
the three units are treated and controlled as a single firing group A with 
a large flash output.
Group Control
Firing group A
To fire the three firing groups A, B, and C at the same time, set <u> 
<h>. With the <u> setting, firing group C does not fire.
If you shoot with firing group C pointing directly toward the main subject, 
overexposure may result.
The flash ratio of 8:1 to 1:1 to 1:8 is equivalent to 3:1 to 1:1 to 1:3 (1/2-stop 
increments) when converted to the number of stops.